MySQL index

MySQL Indexes

The index is equivalent to the catalogue of a Book , Through the directory, you can quickly find the corresponding resources .

In terms of databases , There are two retrieval methods when querying a table ：
The first way ： Full table scan
The second way ： Search by index （ It's very efficient ）

					 The fundamental principle is to narrow the scanning range .

Although index can improve retrieval efficiency , But you can't add indexes at will , Because the index is also an object in the database , It also requires constant database maintenance . There are maintenance costs .

such as , The data in the table is often modified, so it is not suitable to add indexes , Because once the data is modified , The index needs to be reordered , For maintenance .

Adding an index is to a field , Or add an index to some fields .

select ename,sal from emp where ename = 'SMITH';
// When ename When no index is added to the field , above sql Statement will perform a full table scan , scanning ename All values in the field .
// When ename When adding an index to a field , above sql Statement will scan... According to the index , Rapid positioning .

When to use index

• The amount of data is huge ( It depends on the demand and online conditions );

• This field is rarely DML operation ( Field modification , Corresponding index maintenance is also required );

• This field often appears in where clause ( Which field do you often query based on );

• Primary key and have unique The constrained fields are automatically indexed ;

  Therefore, the primary key query is more efficient , Try to retrieve according to the primary key

see sql Statement execution plan

explain sql sentence ;

Create index object

create index  The index name  on  Table name ( Field name );

Delete index object

drop index  The index name  on Table name ;

The realization principle of index

The underlying data structure of the index is :B+Tree

Any row record in the database table has a physical address

Fields are sorted and partitioned by index , Then search according to the query conditions , Last return physical address , The actual query statement becomes as follows :

select ename from emp where  Physical address ;

adopt B Tree Narrow the scanning range , The underlying index is sorted , Partition , The index will carry the data in the table “ Physical address ”,
After the data is finally retrieved through the index , Get the associated physical address , Locate the data in the table by physical address , efficiency
The highest. .
select ename from emp where ename = ‘SMITH’;
Convert to by index ：
select ename from emp where Physical address = 0x3;

Classification of indexes

Single index : Add an index to a single field ;

Composite index : Add an index to the union of multiple fields ;

primary key : The primary key automatically adds an index ;

unique index : Yes unique The constrained fields are automatically indexed ;

…

When the index expires

When fuzzy query , The first wildcard uses %, At this time, the index is invalid .